Giant Conduction at 180° Domain Walls in Tetragonal PZT Controlled by Monolayer 2D Graphene Electrodes for Memristive Applications

Conductive domain walls (DWs) in ferroelectrics offer a promising concept for future memristive nanoelectronics in which these 2D channels function either as the nanoscale information storage units or as synaptic interconnections. The progress towards applications is impeded by relatively low DW conduction in the picoampere range as well as a lack of a precise control of the DW position.<br/>Here we present highly conductive domain-wall-based device concept using the well-studied material Pb(ZrTi)O₃ (PZT) in combination with monolayer patterned graphene electrodes. The artificially created 180° domain walls in our strained 60nm PZT films (deposited on a DyScO₃ substrate with a SrRuO₃ bottom electrode) exhibit non-thermally activated currents of 200-400nA/μm for voltages ≤2V. They are created by electrically poling with moderate voltages of ~5V and show excellent stability enhanced by adjacent stable a- and c-domain configurations. Our recent results (Ref.1) suggest that the anomalously high conduction originates from the interplay between the interconnected a-domains and c-domains during the poling process in these thin films. This leads to the formation of nested and non-vertical 180° domain walls in the proximity of the bottom interface as well as to the distortion of a-domains, which break the domain boundary neutrality. First predicted by simulations, recent TEM studies now experimentally prove these non-trivial interdependent structures involving 180° domain walls and a-domains which lead to highly charged interfaces within the film, responsible for the extraordinary conductive response. Vertical parallel-plate capacitor devices using standard Cr/Au electrodes were tested, forming deterministically-switchable non-volatile resistive states with on/off ratios of 10³ and tens of nanoampere read-out currents at 2V with excellent retention and stability characteristics. These functionalities were enabled by movement of a domain wall through an applied electric field. Moving towards more advanced functionalities, graphene flake electrodes were employed, allowing for a set of different resistive levels within one device by injecting and removing of more than one conductive domain wall under the electrode. More precise injection control is further offered by using patterned monolayer graphene devices as well as exploiting pressure induced conductive tuning. Thus, the multistate memristive devices are enabled by injection of the highly conductive domain walls, bridging the two electrodes of the capacitor, in combination with modified internal structure of the c-/a-domain interconnections, which gradually alter the conductive response.<br/>The authors gratefully acknowledge funding from the EU Horizon 2020 program under the MSCA-ITN action MANIC.<br/><br/>Reference:<br/>(1) F. Risch el al., Nature Communications, 13, No 7239 (2022)